Integrated circuits may be formed using photolithography processes with illumination sources having wavelengths more than twice a desired pitch of metal interconnect lines in the integrated circuits. Attaining desired tradeoffs between fabrication costs and fabrication yield may be difficult. For example, standard single photoresist patterns begin to blur at about the 45 nm feature size and 100 nm pitch (feature size plus space between features) when printing with 193 nm wavelength light.
Double patterning technology (DPT), illustrated in FIGS. 1A-1B, may be used to print patterns with a pitch (geometry width plus space width) that is tighter than can be printed with a single exposure. The pattern in FIG. 1A is composed of the metal1 pattern in FIG. 1B, the via 1 pattern in FIG. 1C and the metal2 pattern in FIG. 1D. In DPT technology, as shown FIG. 1B the metal 1 interconnect pattern with a tight single exposure pitch 20 may be decomposed into two photomasks with alternating interconnect rows DPT-M1-A on one DPT photomask with relaxed pitch 22 and alternating interconnect rows DPT-M1-on a second DPT photomask with relaxed pitch 24. For example, a pattern with 100 nm pitch which prints blurred when all geometries are placed on a single photo mask may be decomposed into two DPT photo masks each with a 200 nm pitch which print without blurring.
As shown in FIG. 1C, unlike the metal 1 pattern which has a tight pitch 20 in only the vertical direction, the via pattern has a tight pitch in both the x 34 and the y 36 directions. To print tight pitches in both the x and the y directions would require generating DPT photomasks with reduced pitch in both directions and so would require the generation of 4 DPT photomasks, one with geometries 26, one with geometries 28, one with geometries 30, and one with geometries 32 as shown in FIG. 1C. This is very expensive. In addition, each photomask is aligned separately each with an alignment error. With two DPT photomasks there are two alignment errors. With four photomasks there would be 4 alignment errors significantly increasing yield loss.
As shown in FIG. 2, the photomask for via pattern 30 may have alignment errors in the negative x and positive y directions and the photomask for via pattern 32 may have an alignment error in the positive x direction. Via misalignment may significantly decrease the contact area, 34 and 36, between the via and the underlying metal 1 and between the via and the overlying metal 2 increasing the via resistance and significantly reducing yield.